at 6:22 PM, Andy Lutomirski <l...@amacapital.net> wrote: > >> On Oct 17, 2018, at 5:54 PM, Nadav Amit <na...@vmware.com> wrote: >> >> It is sometimes beneficial to prevent preemption for very few >> instructions, or prevent preemption for some instructions that precede >> a branch (this latter case will be introduced in the next patches). >> >> To provide such functionality on x86-64, we use an empty REX-prefix >> (opcode 0x40) as an indication that preemption is disabled for the >> following instruction. > > Nifty! > > That being said, I think you have a few bugs. First, you can’t just ignore > a rescheduling interrupt, as you introduce unbounded latency when this > happens — you’re effectively emulating preempt_enable_no_resched(), which > is not a drop-in replacement for preempt_enable(). To fix this, you may > need to jump to a slow-path trampoline that calls schedule() at the end or > consider rewinding one instruction instead. Or use TF, which is only a > little bit terrifying…
Yes, I didn’t pay enough attention here. For my use-case, I think that the easiest solution would be to make synchronize_sched() ignore preemptions that happen while the prefix is detected. It would slightly change the meaning of the prefix. > You also aren’t accounting for the case where you get an exception that > is, in turn, preempted. Hmm.. Can you give me an example for such an exception in my use-case? I cannot think of an exception that might be preempted (assuming #BP, #MC cannot be preempted). I agree that for super-general case this might be inappropriate.
